Field of the Invention
The present invention relates to an image-forming apparatus, such as a copying machine or printer, which is provided with a function for forming an image on a recording material.
Description of the Related Art
Conventionally, for example, in an image-forming apparatus using an electrophotographic system, a toner image is formed as a developer image on a photosensitive member which serves as an image bearing member. A transfer step is then performed in which an electric field of opposite polarity to the normal charging polarity of the toner is applied to a transfer device, and the toner image borne on the photosensitive member is transferred electrostatically onto a recording material conveyed on a conveyance belt or the surface of an intermediate transfer belt.
Therefore, in an image-forming apparatus provided with a movable transfer belt, intermediate transfer belt, or the like, a voltage application device which applies a voltage required in the transfer step is provided in the transfer device. In this example, there is a configuration in which a contact transfer member, such as a transfer roller, connected to a high-voltage power source forming a voltage application device, is arranged as a transfer device, at a position opposing the photosensitive member across from the belt (on the rear surface side of the belt).
However, in an image-forming apparatus which uses a transfer roller, there is a problem in that the shape of the contact region (transfer nip section) between the belt and the transfer roller is difficult to control.
In order to resolve a problem of this kind, Japanese Patent Application Publication No. 2009-48051 discloses a configuration in which the transfer member described above is formed by a sheet member, an elastic member for pressing the sheet member against the image bearing member, and a supporting member which supports the sheet member, wherein the sheet member approaches the transfer nip section at a certain angle of inclination by the supporting member. According to this composition, the transfer member makes stable contact with the belt and the transfer electric field can be stabilized.
In general, the device for applying a transfer voltage to the transfer member applies a voltage by making contact with the transfer member by means of a metal contact member, or the like.
However, there is a concern in that since the metal has a much lower electrical resistance value than that of the material generally used as the transfer member, then there are local variations in the transfer electric field peripheral to the electric contact point, and the image in this portion is disturbed. In particular, in a configuration in which a sheet member approaches a transfer nip section with a certain angle of inclination, as disclosed in Japanese Patent Application Publication No. 2009-48051, then the transfer electric field is liable to be affected in the peripheral area where the sheet member approaches the transfer nip section. Furthermore, metal has a high coefficient of elasticity with respect to the elastic materials and the sheet members which are generally used as transfer members. Consequently, there is a concern in that the coefficient of elasticity of the transfer member varies in the peripheral area where the contact member is connected, the transfer nip section displays local instabilities, and hence the transfer characteristics are impaired.